Submarine sound detection



E. MEBRITT.

SUBMAR|NE SOUND DETECTION.

- APPLICATION FILED SEPT. 6, 1919.

' Patented May 17, 1921.

2 SHEETS SHEET I.

1 INVENTOR.

E. MEHRJTT.

SUBMARINE SOUND DETECTION.

APPLICATION FILED SEPT. 6. 1919;

1,878,420. Patented May 17,1921,

2 SHEETS-SHEET 2.

INVENTOR.

UNITED STATES,

PATENT OFFICE.

mmns'r Murmur, or rrnaca, NEW-YORK.

SUBMARINE SOUND DETECTION.

To all whom, it 12mg} concern Be it known that I, EnNns'r Mnnnrrr, a citizen of the United States, residing at Ithaca, in the county of Tompkins and State of New York, have invented new and useful Improvements in- Submarine Sound Detection, of which the following is a specification.

The present invention relates to submarine sound detection and more especially to so called pressure release surfaces and the combination with them of inertia plates.

Referring to the drawings, Figures 1 and 2 are aperspective view and a vertical cross section of a simple form of submarine sound screen. Figs. 3 and 4 are afiperspective view and vertical cross section showing an inertia plate combined with a submarine sound screen. Fig. 5 is a horizontal section of a submarine sound. direction indicating device. Fig. 6 is a horizontal. section of another type of submarine sound direction indicating device. Fig. 7 is a vertical section showing I an inertia plate secured to a ships hull. Fig. 8 is a vertical section sho ing an inert1a plate and sound screen secured to a'ships hull and covered with a blister. Fig. 9 is a iide elevation of the structure shown in hound waves are readily transmitted through a liquid such as water. Advantage of this is taken by the use of so-called hydrophones or sound receivers placed under water for the detection of other ships particularlysubma'rines. The observer liege-m ing in the receiver can hear the propeller noise of another ship for a considerable distance. This has led to the extensive application of listening devices for the detection of submarines. Becauseof the greater density and elasticity of water as com-pared with air, screen'- ing materials placed in water have a distinctly different behavior from that. of the same materials placed in air. Numerous experiments have shown that solid objects such as plates of iron or leadare not effective as screens when submerged in the water.

,A plate of lead an inch thick and twelve inches square produces. no appreciable screening effect except for the extremely Specification of Letters Patent.

the screen shown are made of thin metal,

Patented May 17, 1921.

Application filed September 6, 1919. I Serial No. 322,229.

high pitched components of the sound. The plates of an iron vessel, even with the added stiffness and inertia of the frames and bulkheads, permit the sound waves to pass so freely from the surrounding water that listening apparatus may be used in the water 1 or oil tanks of the vessel. The extremely small screening effect observed in such cases is doubtless due to' the fact that the thickness of the metal is always small compared to the wave length of audible sounds. With sound Whose pitch is above the audible vibrations, so that the wave length in water is only a few inches, the screening eflectof such metal plates is undoubtedly great, but With sound whose pitch is below three thousand feet' per second it appears-highly improbable that useful screening oan be obtained with masses of metal of such weight and dimensions as would permit of their em- .ployment in practice. On the other hand if we go to the opposite extreme and use ma-. terials which modify the sound waves in the water, not because of their rigidity and inertia, but because of their ability to yield freely to the pressure of the waves, effective screens can be made and it is possible to get useful results by the employment of screens of moderate dimensions. A bubble of air or a toy balloon of thin rubber is perhaps the simplest illustration of a screen of this type. When sound waves fall upon the balloon or bubble, the surface yields so freelv that no appreciable increase in pressure can be pro-- duced in the immediate neighborhood of the ,surface of the balloon or bubble, and the pressure of the' water is therefore unaffected by the sound waves' Such a surface is known as a pressure release surface. A sound receiver whose action depends on pressure change will give no response when placed in the water close to suchv a surface.

Screens which possess this property of yielding freely to ressure may be made in a reat variety 0 ways. For example, a thin rubber bag or pad filled with. air forms a good screen. Also air filled boxes with metal walls make ood screens provided the metal is not too thick. A screen of this type is shown in Figs. 1 and 2. The walls 1 of I such as tin plate', soldered other .water I tightto leaveanair cavity. he inside of the screen shown in .Figs. 1 and 2 is filled with some yielding material such as felt t which prevents the sides from collapsing but permits the walls to yield under the sound waves. The hull of a vessel, even" when the plates are as thick as three quarters of an inch, forms a yielding or pressure release surface for sound waves, as is demonstrated by the fact that the intensity of a sound heard in a submarine receiver placed near the hull is greatly reduced.

If a screen such as an air filledrubber bag 16 or. a yielding wall or air filled metal screen 20 is as close to the screen as it can be placed and increases gradually. as the receiver is moved farther away. If the. distance istwice as great as the diameter of the screen,

the intensity is practically the same as though the screen were absent, because the sound is diffracted around the screen. Thus for example, a receiver 4 as shown in Fig. 2 is shielded by the screen 1 from sound coming from the direction indicated by the arrow '5. This action can be accounted for if we assume thatthe screen absorbs the wayes falllng u on it but it can also be-accounted for equa ly-well upon the assumption that the waves are reflected by the screen. In' 351e1ther case diffraction around the edges" ing'of the surface of the sound screen can Would prevent the sound shadow-from being perfectly defined. On the other hand, if the a receiver 4 is pla'cedbetween the screen 1 and the source of sound coming in the direction 40 of arrow 6, the intensity of the sound heard is also reduced by the presence of the screen, and if the receiver is placed close to the screen the reduction in intensity is practically the same whether the receiver is .in

front or behind. This reduction in intenany sity in front of the screen cannot be accounted for on accountof absorption, which would merel annul the waves falling upon the screen without alteration or reduction in way ,at the points through which the waves" havealready passed.- To account for the reduced intensity i front of the screen it seems necessary to assume that when sound waves fall upon the yielding wall of the screen, secondary waves are sent back from it and the phase relations are such as to. make thesecondary waves partially neutralize the original disturbance. Such action by the screen is in fact to be expected; for as the surface of the screen yields to the incident waves it moves back and forth in exactly such away as would cause it, if incident waves were not present, to radiate sound waves of the same 95 character as the incident sound, The d sdent wave.

turbance at any point near the screen must therefore be regarded as due to the superposition of these secondary waves upon the waves originall present. It is clear that it the surface '0 waves must be equal in amplitude and opposite in phase to the waves which produce them, for their effect at that point is to reduce the pressure change to zero.

shown in Fig. 1 differ in their behavior to i no appreciable extent from an ideal screen in which the yielding is complete.

If the receiver 4 is moved away from the surface ,of the sound screen 1, the loudness of the sound heard increases. If the receiver is moved about twelve inches away from the screen, the sound heard is of about the same loudness as it would be if the'soimd screen were not present. This distance will depend however upon the pitch of the sound, being less for high pitch sounds.

the screen the secondary When the'receiver is closeto the sound screen, the ressure on the water is released b the yiel ing surface of the sound screen.

hen the receiver is moved away from the sound screen for a little distance, the yieldproduce no effect in pressure reduction at the receiver until the inertia of thewater- Y of the screen, the pressure release surfacehas no appreciable effect upon the receiver;

It is therefore ssible to use the receiver 4 for the receptlon of sounds coming from For this reason if there isv the direction indicated by the arrow 6, pro-$ vided the receiver 4 is placed-far enough from the soundscreen 1 to be outside that re'giongin the water. in which there is a. substantial reduction in the pressure variation of the sound waves due to the pressure release surface of the screen. On the other hand, .the farther away thereceiver 4 is from the screen, the less shielded. it is from sounds coming from the direction shown by the arrow 5. It is therefore desirable to place the ,receiver 4 as close to the sound screen as possible. I have found that the re,-

ceiver. 4 may be placed closer to the sound screen if an inertia plate is placed between the'sound screen and the receiver. Referri to Figs. 3 and at, a solid metal plate 10 of ead or iron is placed close to the sound s ee and the receiver i s p d cl se to the plate 10. "Lead has about eleven times the density of water so that a plate of lead one inch thick has substantially the same inertia as a layer of water eleven inches thick. If for the inertia of the water is substituted the inertia of such a plate, the receiver 4 may be placed close to the sound screen 1 with the plate 10 interposed between the receiver and sound screen. It has been found that when a receiver was mounted immediately in front of a lead plate about fourteen inches square and one inch thick, the sound heard in the receiver was substantially the same whether the screen was placed behind the lead plate or not.

The use of the inertia plate 10 as shown in Fig. 4 allows the same intensity of sound coming from the direction indicated by the arrow 6 to be heard at the receiver 4 placed close to the screen as when the receiver '4 is placed at a considerable distance from the screen as shown in Fig. 2. On the other hand, the receiver 4 placed closed to the screen as shown in Fig. 4 is much more effectively shielded from sound coming from a direction indicated by arrow 5. Sound screens of the yielding wall type, which cut off substantially all'sound by reflection may be used in the construction of sound direction indicating devices. If a cup shaped screen like that shown at 20 in Fig. 5 partially surrounds a receiver 21, receiver 21 will be shielded from sounds coming from the directions indicated by the arrows 22 but will not be shielded from sounds coming from, the direction indicated by the arrow 23, so that if the screen 20 and the receiver 21 are turned around in the Water, the direction of the sound may be determined by its maximum as heard in the receiver 21. A sound screen 20 may be used alone with a receiver 21 provided the sound screen is large enough so that the receiver 21 does not have to be placed too near the pressure release surface of the screen. If an inertia plate 24 is interposed between the receiver 21 and the screen 20, the receiver 21 may be placed close to the screen. 7 This permits a smaller screen to be used and permits the receiver 21 to be placed near the bottom of the cup-like cavity formed by thev screen. This permits a smaller device to be used and gives sharper indication of direction.

Another device employing a sound screen for detecting the direction of sound is I shownin Fig. 6. In this figure, two receivers '30 and 31 are connected to ear pieces 32 and 33 respectively so that the direction of the incident sound waves may be determined by binaural sensation. A sound screen 34 is shown screening the receivers 30 and 31 from sounds from behind. This screen has an extension 35 between the receivers. It is found that the effect of the screen 35-between the receivers is to make the binaural bodied in other structures within the immersed in a liquid and a sound screen sensation more perfectly defined for the same distance of separation between the,receivers 30 and 31. Forexample, if the receivers 30 and 31 are eighteen inches apart in the water with no sound screen between them the binaural effect is not sharply defined, but if a sound screen is placed between the two receivers the binaural sensation is as sharply defined as though the distance had been increased to three feet or more. lVhen a sound screen is used. as shown in Fig. 6 to separate the receivers, inertia plates 36 are preferably used as indicated.

In Fig. 7 is shownthe application of an inertia plate 40 to the side ofa ship. The ships plates 41, as above explained, operate as a pressure-release surface because being backed up with air they yield to the sound waves. If a .hydrophone or submarine sound receiver is placed too close to the side of the ship the intensity of the sound 18 greatly diminished. However, if a heavy plate, such as a plate of lead or iron an inch or .,more thick, be secured to the side of the ship as shown at 40 in Fig. 7, the sound receiver 41 may be placed closeto the ships hull,

In Figs. 8 and 9 is shown an arrangement of a sound screen, inertia plate and s'ubmarine sound receiver for use on a ships hull. A sound screen 50 is placed on the ships hull so as to be surrounded by water, and screens the receiver 51 from the sounds produced in the water by the vibrations of the hull, such vibrations being due, for example, to the ships engines or to waves beating against the hull. An inertia plate 52 1S placed between the receiver 51 and the plate 50. This permits the receiver to be placed close to thescreen 50 to get a maxlmum screening effect. -F or structural reasons 1t is also advantageous to have the recelver as close to the side of the ship as possible. The soundscreen 50,. the inertia plate 52 and microphone 51 are secured tothe side of the ship and may to advantage be 1nclosed as a whole by a curved stream line 1 plate 53 known as a blister.

Thepresent invention is not limited to its illustrated embodiment but may be emscope of the following claims.

I claim: I

l. The combination of a sound screen hav- 1 20 ing a yielding pressure release surface immersed in a liquid, a'sound receiver immersed inthe liquid in proximity to the screen, and an inertia plate interposed between the screen and receiver, substantially as described.

2. The combination of a sound receiver ar-. tially surrounding the receiver and shlelding it from sounds at back and sides, and

an inertia plate between the receiver and sound screen, substantially as described.

3. The combination of a pressure release surface exposed to a sound conducting liq- 5 ui'd, a sound receiver in the liquid in close .the sound waves if no added inertia mass, were. interposed between the surface and re-;

ceiver, and an inertia. mass interposed "between the surface and' the receivers where- .by -the sound pressure'at the receiver is increased, substantially as described.

ERNEST MERRITT. 

